A customary process for producing and processing aqueous concrete compositions is the dry spray concrete process, in which dry concrete mixes are made up with water directly on the construction site. In the dry spray concrete process, the dry concrete mix is conveyed, for example pneumatically, through a transport line to a spray nozzle and made up with water and, if appropriate, mixed with further additives in the spray nozzle. The concrete compositions are thus sprayed onto a substrate by means of the spray nozzle immediately after making up with water. Dry concrete mixes comprise cement, fillers such as sand or gravel and, if appropriate, setting accelerators and polymers and also further additives. The addition of polymers enables the processability of concrete compositions or the property profile of concrete to be improved, for example its adhesion to various substrates, gas permeability, E modulus, or hydrophobicization of the concrete can be achieved. Setting accelerators such as aluminum sulfate considerably accelerate the curing of aqueous concrete compositions and thereby produce better resistance to sagging of the aqueous concrete compositions on their substrate. This simplifies the processing of aqueous concrete compositions and makes vertical or overhead processing possible. Depending on requirements, stiffening times of from a number of minutes to a few seconds can be set by appropriate use of setting accelerators.
The dry spray concrete process is particularly useful when using relatively small amounts of concrete compositions, as is frequently the case, for example, in the renovation of building works.
However, a fundamental problem is that aqueous concrete compositions having different contents of setting accelerators and polymers are frequently required in different construction sections of the same construction site. The required adaptation of the composition of the aqueous concrete compositions has, however, hitherto not been possible in a practicable and economical way. Previously known compositions containing setting accelerators and polymers are storage-stable only in dry form. However, the entire dry concrete mix would have to be mixed on addition of polymers or setting accelerators, which would be associated with a large outlay in terms of time, apparatus and logistics. It would be more efficient to mix setting accelerators and polymers into the water with which the dry concrete mixes are made up in the spray nozzle. However, this is not possible since previously known setting accelerators and polymers immediately coagulate and flocculate in aqueous form and therefore have to be applied immediately after mixing without a further intermediate step; i.e. mixing of setting accelerators and polymers could hitherto only be carried out in aqueous concrete compositions and this in turn only immediately before application to a substrate.
Overall, the dry spray concrete process is thus not very flexible in respect of the production of aqueous concrete compositions containing variable amounts of setting accelerators and polymers. Furthermore, a large amount of dust is produced in the handling of dry concrete mixes, and this has an adverse effect on the health of the workers on the construction site and makes complicated safety precautions necessary.
The use of wet concrete mixes, in particular using the wet spray concrete process, is also widespread. Wet concrete mixes comprise cement, fillers such as sand or gravel and, if appropriate, polymers and further additives and have already been made up with water. In the wet spray concrete process, setting accelerators are added to the wet concrete mixes in the spray nozzle, i.e. immediately before application to the respective substrate. It is advantageous that the water content, the consistency and thus the quality of the wet spray concrete can be set precisely when using the wet spray concrete process. The wet spray concrete process is particularly suitable when relatively large amounts of concrete compositions are employed, as is the case, for example, in tunnel construction.
However, the wet spray concrete process also has the disadvantage that the amount of polymers in the wet concrete mixes cannot be varied on the construction site without a further, complicated mixing step. This is because on addition of polymers to a wet concrete mix, the entire wet concrete mix would have to be mixed, which in view of the large quantities of aqueous concrete compositions to be processed would be associated with a considerable outlay in terms of time and apparatus and additionally represent a considerable change to the established logistics on a construction site.